1. Field of the Invention
The present invention relates generally to blinds and, more specifically, to a lift lock used in a blind to lock the lift rod.
2. Description of the Related Art
Conventional vertically adjustable blinds are numerous, including Venetian blinds, Roller blinds, Pleated blinds, Honeycomb shades, Roman blinds, and etc. An adjustable blind generally comprises a headrail fixedly fastened to the top of the window, a bottom rail spaced below the headrail, a blind body (formed of a set of blind slats, a piece or curtain, or pleated slats) connected between the headrail and the bottom rail, and a lift cord suspended from the headrail at one lateral side for pulling by hand to lift or lower the bottom rail and the blind body. There are motor-driven blinds that use a motor drive to lift/lower the bottom rail. Because the suspending part of the lift cord is exposed to the outside of the headrail and easily accessible by a child, the suspending part of the lift cord may be hung on a child""s head accidentally. In order to eliminate this problem, blinds with hidden lift cord are disclosed. A blind with hidden lift cord comprises a lift rod fastened pivotally with the inside of the headrail, and at least one spring member (normally, torsional spring) mounted inside the headrail and coupled to the lift rod. The lift rod can be rotated clockwise or counter-clockwise to roll up or let off the lift cord, so as to further lift or lower the bottom rail of the blind. The spring power of the spring member bears the weight of the bottom rail as well as the blind slats and is maintained in balance with the torque of the lift rod, enabling the blind to be positioned in the desired extending position. During operation, the user needs only to give an upward or downward pressure to break the balance.
The aforesaid structure of using the spring power of a spring member to support the lift rod at the desired elevation is functional. However, this design still has drawback. One drawback is the difficulty of accurately controlling the spring power of the spring member during installation (The size of a blind may have to be adjusted subject to the size of the window in which the blind is to be installed). Another drawback of this design is that the spring power of the spring member deteriorates with the use of the blind. When the spring power of the spring member changed, the blind tends to be lowered slightly after pulled to the desired elevation, and the touch of a small (unexpected) external force may cause the blind to lift or lower the bottom rail for a distance.
Therefore, it is desirable to provide a lift lock for blind that eliminates the aforesaid drawbacks.
It is the main object of the present invention to provide a lift lock for blind, which enables the user to control the elevation of the blind body conveniently and accurately without the use of an exposed pull cord and, which accurately locks the blind body in position when adjusted.
To achieve this object of the present invention, the lift lock for blind comprises a shaft fastened pivotally with the inside of a headrail of a blind for synchronous rotation with a lift rod of the blind. The shaft has a constraint device. A switching mechanism is mounted inside the headrail of the blind and has a sliding member movable between a locking position and an unlocking position. The sliding member includes an engagement device, which is forced into engagement with the constraint device to stop the shaft from rotary motion when the sliding member moved to the locking position, or disengaged from the constraint device for enabling the shaft to be rotated by an external force when the sliding member moved to the unlocking position. A control mechanism has a vertical pull rod provided with a top end suspended from one end of the headrail and coupled to the switching mechanism for pulling downwardly by the user to switch the switching mechanism between the locking position and the unlocking position alternatively. And, a return spring is mounted in the headrail and adapted to return the pull rod each time the pull rod been pulled downwards by the user and released.